Epitaxial substrate for semiconductor elements, semiconductor element, and manufacturing method for epitaxial substrates for semiconductor elements

ABSTRACT

An epitaxial substrate for semiconductor elements suppresses leakage current and has a high breakdown voltage. The epitaxial substrate for semiconductor elements includes: a semi-insulating free-standing substrate formed of GaN doped with Zn; a buffer layer formed of a group 13 nitride adjacent to the free-standing substrate; a channel layer formed of a group 13 nitride adjacent to the buffer layer; and a barrier layer formed of a group 13 nitride on an opposite side of the buffer layer with the channel layer therebetween, wherein part of a first region consisting of the free-standing substrate and the buffer layer is a second region containing Si at a concentration of 1×1017 cm−3 or more, and a minimum value of a concentration of Zn in the second region is 1×1017 cm−3.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of International Application No.PCT/JP2016/079616, filed Oct. 5, 2016, which claims the benefit of U.S.Provisional Application No. 62/249,537, filed Nov. 2, 2015 and JapaneseApplication No. 2016-005115, filed Jan. 14, 2016, all of which arehereby incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a semiconductor element, andparticularly to a semiconductor element constituted by using afree-standing substrate formed of semi-insulating GaN.

BACKGROUND ART

Nitride semiconductors, which have a direct-transition-type wide bandgap, high breakdown electric field, and high saturation electronvelocity, have been used as light emission devices such as LED or LD andsemiconductor materials for high-frequency/high-power electronicdevices.

Typical structures of the nitride electronic devices include a highelectron mobility transistor (HEMT) structure which is formed bylaminating AlGaN as “a barrier layer” and GaN as “a channel layer”. Thisstructure utilizes a feature that a high concentration two-dimensionalelectron gas is generated at an AlGaN/GaN lamination interface owing tolarge polarization effects (spontaneous polarization effect and piezopolarization effect) inherent in nitride materials.

The nitride electronic devices are generally manufactured usingdifferent material base substrates such as sapphire, SiC, and Si whichare easily available in a commercial way. However, there arises aproblem that large numbers of defects occur in a GaN film which isheteroepitaxially grown on the different material substrates due to adifference in lattice constant and heat expansion coefficient betweenGaN and the different material substrates.

In the meanwhile, when the GaN film is homoepitaxially grown on a GaNsubstrate, the defect caused by the difference in lattice constant andheat expansion coefficient described above does not occur, but the GaNfilm shows a favorable crystalline nature.

Accordingly, when the nitride HEMT structure is manufactured on the GaNsubstrate, mobility of the two-dimensional electron gas at the AlGaN/GaNlamination interface is enhanced, thus a characteristic improvement ofan HEMT element (semiconductor element) manufactured using the abovestructure can be expected.

However, the GaN substrate manufactured by a hydride vapor phaseepitaxial growth method (HVPE method), which can be commerciallyavailable, generally has an n-type conductivity due to an oxygenimpurity incorporated into a crystal. The conductive GaN substrateserves as a leakage current pathway between source-drain electrodes whenthe HEMT element is driven at high voltage. Thus, it is preferable touse the semi-insulating GaN substrate to manufacture the HEMT element.

It is known to be efficient to perform doping of an element such as atransition metal element (Fe, for example) or group 2 element (Mg, forexample) which forms a deep acceptor level in the GaN crystal to achievethe semi-insulating GaN substrate.

It is already known that when zinc element (Zn) is adopted from a group2 element, a high-quality semi-insulating GaN single-crystal substratecan be achieved (for example, refer to Patent Document 1). Also known isan aspect that a high-resistance layer doped with iron (Fe), which is atransition metal element, is formed on a substrate, and an intermediatelayer having a high effect of incorporating Fe is further formed betweenthe high-resistance layer and an electron transit layer, therebypreventing Fe from being incorporated into the electron transit layer(for example, refer to Patent Document 2).

A manufacture of the HEMT structure on the semi-insulating GaN substrateor a substrate with the semi-insulating GaN film to evaluate eachcharacteristic has been already performed (for example, refer toNon-Patent Document 1 to Non-Patent Document 3).

When the nitride film is epitaxially grown on the semi-insulating GaNsubstrate, silicon (Si) element may be incorporated into an interfacebetween the semi-insulating GaN substrate and the nitride film (nitrideepitaxial film) from outside. Since the silicon (remaining silicon)serves as a donor element, a conductive layer is generated at thenitride film/substrate interface. Since the conductive layer serves as aleakage pathway of drain-source current in the HEMT element, therebycausing a reduction in pinch-off characteristics and breakdown voltage.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent No. 5039813

Patent Document 2: Japanese Patent Application Laid-Open No. 2013-74211

Non-Patent Documents

Non-Patent Document 1: Yoshinori Oshimura, Takayuki Sugiyama, KenichiroTakeda, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, IsamuAkasaki, and Hiroshi Amano, “AlGaN/GaN Heterostructure Field-EffectTransistors on Fe-Doped GaN Substrates with High Breakdown Voltage”,Japanese Journal of Applied Physics, vol. 50 (2011), p. 084102-1-p.084102-5.

Non-Patent Document 2: V. Desmaris, M. Rudzinski, N. Rorsman, P. R.Hageman, P. K. Larsen, H. Zirath, T. C. Rodle, and H. F. F. Jos,“Comparison of the DC and Microwave Performance of AlGaN/GaN HEMTs Grownon SiC by MOCVD With Fe-Doped or Unintentionally Doped GaN BufferLayers”, IEEE Transactions on Electron Devices, Vol. 53, No. 9, pp.2413-2417, September 2006.

Non-Patent Document 3: M. Azize, Z. Bougrioua, and P. Gibart,“Inhibition of interface pollution in AlGaN/GaN HEMI structures regrownon semi-insulating GaN templates”, Journal of Crystal Growth vol. 299(2007) p. 103-p. 108.

SUMMARY

The present invention relates to a semiconductor element, andparticularly to a semiconductor element constituted by using afree-standing substrate formed of semi-insulating GaN.

According to the present invention, an epitaxial substrate forsemiconductor elements includes: a semi-insulating free-standingsubstrate formed of GaN doped with Zn; a buffer layer formed of a group13 nitride adjacent to the free-standing substrate; a channel layerformed of a group 13 nitride adjacent to the buffer layer; and a barrierlayer formed of a group 13 nitride on an opposite side of the bufferlayer with the channel layer therebetween, wherein part of a firstregion consisting of the free-standing substrate and the buffer layerincludes a second region containing Si at a concentration of 1×10¹⁷ cm⁻³or more, and a minimum value of a concentration of Zn in the secondregion is 1×10¹⁷ cm⁻³.

According to another aspect of the present invention, a semiconductorelement includes: a semi-insulating free-standing substrate formed ofGaN doped with Zn; a buffer layer formed of a group 13 nitride adjacentto the free-standing substrate; a channel layer formed of a group 13nitride adjacent to the buffer layer; a barrier layer formed of a group13 nitride on an opposite side of the buffer layer with the channellayer therebetween; and a gate electrode, a source electrode, and adrain electrode provided on the barrier layer, wherein part of a firstregion consisting of the free-standing substrate and the buffer layer isa second region containing Si at a concentration of 1×10¹⁷ cm⁻³ or more,and a minimum value of a concentration of Zn in the second region is1×10¹⁷ cm⁻³.

According to still another aspect of the present invention, a method ofmanufacturing an epitaxial substrate for semiconductor elementsincludes: a) a preparation step of preparing a semi-insulatingfree-standing substrate formed of GaN doped with Zn; b) a buffer layerformation step of forming a buffer layer formed of a group 13 nitrideadjacent to the free-standing substrate; c) a channel layer formationstep of forming a channel layer formed of a group 13 nitride adjacent tothe buffer layer; and d) a barrier layer formation step of forming abarrier layer formed of a group 13 nitride in a position opposite to thebuffer layer with the channel layer therebetween, wherein a secondregion containing Si at a concentration of 1×10¹⁷ cm⁻³ or more is formedthrough taking Si in said free-standing substrate, which has beenprepared in said preparation step, from outside before said buffer layerformation step is completed, in part of a first region consisting of thefree-standing substrate and the buffer layer, and in the buffer layerformation step, Zn is diffused from the free-standing substrate, therebyforming the buffer layer in which a minimum value of a concentration ofZn is 1×10¹⁷ cm⁻³ in the second region.

According to the present invention, the leakage current is reduced indriving the semiconductor element, and breakdown voltage (elementvoltage) of the semiconductor element can be enhanced.

Thus, it is an object of the present invention to provide an epitaxialsubstrate for semiconductor elements which suppresses a leakage currentand has a high breakdown voltage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing schematically illustrating a cross-sectionalstructure of an HEMT element 20.

FIG. 2 is a drawing illustrating a concentration profile of Zn elementand Si element in a neighborhood of an interface between a GaN bufferlayer and a GaN substrate in an Example 1.

FIG. 3 is a drawing illustrating a concentration profile of Zn elementand Si element in a neighborhood of an interface between a GaN bufferlayer and a GaN substrate in a Comparative Example 1.

FIG. 4 is a drawing illustrating a concentration profile of Zn elementand Si element from a surface of a barrier layer 4 in a depth directionand a secondary ion signal profile of Al element in an Example 7.

DESCRIPTION OF EMBODIMENT(S)

Group numbers of a periodic table in the present specification areaccording to the explanation of group numbers 1 to 18 in thenomenclature of inorganic chemistry revised in 1989 by the internationalunion of pure applied chemistry (IUPAC). Group 13 refers to, forexample, aluminum (Al), gallium (Ga), and indium (In), group 14 refersto, for example, silicon (Si), germanium (Ge), tin (Sn), and lead (Pb),and group 15 refers to, for example, nitrogen (N), phosphorous (P),arsenic (As), and antimony (Sb).

<Summary of Epitaxial Substrate and HEMT Element>

FIG. 1 is a drawing schematically illustrating a cross-sectionalstructure of an HEMT element 20 as one embodiment of a semiconductorelement according to the present invention, which includes an epitaxialsubstrate 10 as one embodiment of an epitaxial substrate forsemiconductor elements according to the present invention.

The epitaxial substrate 10 includes a free-standing substrate 1, abuffer layer 2, a channel layer 3, and a barrier layer 4. The HEMTdevice 20 comprises a source electrode 5, a drain electrode 6, and agate electrode 7 disposed on the epitaxial substrate 10 (on the barrierlayer 4). The ratios of the respective layers in FIG. 1 do not reflectthe actual ones.

The free-standing substrate 1 is a GaN substrate doped with 1×10¹⁸ cm⁻³or more of Zn and has a (0001) plane orientation, a resistivity of 1×10²Ωcm or more at room temperature and semi-insulation properties. Althoughthe size of the free-standing substrate 1 is not particularly limited,the free-standing substrate 1 preferably has a thickness ofapproximately several hundreds of μm to several mm in consideration of,for example, ease of handling. The free-standing substrate 1 can bemanufactured by a flux method, for example.

The free-standing substrate 1 formed by the flux method is obtained bythe following processes briefly of: immersing a seed substrate in a meltcontaining metal Ga, metal Na, metal Zn, and C (carbon) in a growingvessel (alumina crucible) disposed to be horizontally rotatable in apressure vessel; keeping a predetermined temperature and a predeterminedpressure in the growing vessel with the introduction of nitrogen gas,while horizontally rotating the growing vessel; and then separating aGaN single crystal, which is resultantly formed on the seed substratefrom the seed substrate. A so-called template substrate in which a GaNthin film is formed on a sapphire substrate by a MOCVD method can bepreferably used as the seed substrate.

The buffer layer 2 is a layer formed of a group 13 nitride, (adjacently)formed on one main surface of the free-standing substrate 1. The bufferlayer 2 may be a single layer which is wholly formed of a group 13nitride or may be a multi-layered buffer layer which is formed of two ormore group 13 nitride layers having different compositions. Examples ofthe single layer include, for example, a GaN buffer layer which iswholly made of GaN. Examples of the multi-layered buffer layer include,for example, a configuration that a GaN layer is laminated on aAl_(a)Ga_(1-a)N layer (0<a≤1). Alternatively, the buffer layer 2 may beprovided as a composition gradient buffer layer formed of a group 13nitride containing two or more group 13 elements (for example, Ga andAl), each element having an existence ratio (mole fraction) changed in athickness direction. The buffer layer 2 is formed to have a thickness ofapproximately 50 to 1000 nm. In the present embodiment, the buffer layer2 is formed at a temperature substantially the same as or higher than aformation temperature of the channel layer 3 and the barrier layer 4,differing from a so-called low-temperature buffer layer formed at a lowtemperature lower than 800° C.

In the epitaxial substrate 10 according to the present embodiment, Zn,with which the free-standing substrate 1 is doped, is diffused into atleast the buffer layer 2. This point is described hereinafter.

The channel layer 3 is a layer (adjacently) formed on the buffer layer2. The channel layer 3 is formed to have a thickness of approximately 50to 5000 nm. The barrier layer 4 is a layer provided on an opposite sideof the buffer layer 2 with the channel layer 3 therebetween. The barrierlayer 4 is formed to have a thickness of approximately 2 to 40 nm.

The barrier layer 4 may be formed adjacent to the channel layer 3 asillustrated in FIG. 1, and in this case, an interface therebetween is ahetero junction interface. Alternatively, a spacer layer not shown maybe provided between the channel layer 3 and the barrier layer 4, and inthis case, a region from an interface between the channel layer 3 andthe spacer layer and an interface between the barrier layer 4 and thespacer layer is a hetero junction interface region.

In any case, as a preferred example, the channel layer 3 is formed ofGaN, and the barrier layer 4 is formed of AlGaN (Al_(x)Ga_(1-x)N, 0<x<1)or InAlN (In_(y)Al_(1-y)N, 0<y<1). However, a combination of the channellayer 3 and the barrier layer 4 is not limited thereto.

The formation of the buffer layer 2, the channel layer 3, and thebarrier layer 4 is achieved by the MOCVD method, for example. In a casewhere the buffer layer 2 and the channel layer 3 are formed of GaN andthe barrier layer 4 is formed of AlGaN, for example, the layer formationby the MOCVD method can be performed, using a publicly known MOCVDfurnace capable of supplying an organic metal (MO) source gas for Ga andAl (TMG and TMA), ammonia gas, hydrogen gas, and nitrogen gas into areactor, by heating the free-standing substrate 1 disposed in thereactor to a predetermined temperature and depositing a GaN crystal andan AlGaN crystal generated by a gas phase reaction between the organicmetal source gas corresponding to each layer and the ammonia gas on thefree-standing substrate 1 in sequence.

The source electrode 5 and the drain electrode 6 are metal electrodeseach having a thickness of approximately ten and several nm to a hundredand several tens of nm. The source electrode 5 and the drain electrode 6are preferably formed as multi-layered electrodes of, for example,Ti/Al/Ni/Au. The source electrode 5 and the drain electrode 6 have ohmiccontact with the barrier layer 4. The source electrode 5 and the drainelectrode 6, as a preferred example, are formed by a vacuum evaporationmethod and a photolithography process. It is preferable to perform athermal treatment for several tens of seconds in a nitrogen gasatmosphere at a predetermined temperature of 650 to 1000° C. afterforming the electrodes 5 and 6 to enhance the ohmic contact of thoseelectrodes.

The gate electrode 7 is a metal electrode having a thickness ofapproximately ten and several nm to a hundred and several tens of nm.The gate electrode 7 is preferably formed as a multi-layered electrodeof, for example, Ni/Au. The gate electrode 7 has Schottky contact withthe barrier layer 4. The gate electrode 7, as a preferred example, isformed by a vacuum evaporation method and a photolithography process.

<Method of Manufacturing Epitaxial Substrate and HEMT Element>

(Manufacture of Free-Standing Substrate)

A procedure of manufacturing the free-standing substrate 1 by the fluxmethod is firstly described.

Firstly, a c-plane sapphire substrate having a diameter substantiallythe same as that of the free-standing substrate 1 to be manufactured isprepared, and a GaN low-temperature buffer layer is formed on a surfaceof the c-plane sapphire substrate to have a thickness of approximately10 to 50 nm at a temperature of 450 to 750° C. Subsequently, a GaN thinfilm having a thickness of approximately 1 to 10 μm is formed by theMOCVD method at a temperature of 1000 to 1200° C., thereby obtaining aMOCVD-GaN template usable as a seed substrate.

Next, a Zn-doped GaN single crystal layer is formed by a Na flux methodusing the MOCVD-GaN template which has been obtained as the seedsubstrate.

Specifically, the MOCVD-GaN template is firstly disposed in an aluminacrucible, and subsequently, the alumina crucible is filled with 10 to 60g of metal Ga, 15 to 90 g of metal Na, 0.4 to 5 g of metal Zn, and 10 to500 mg of C.

The alumina crucible is put in a heating furnace and heated forapproximately 20 to 400 hours with a furnace temperature of 800 to 950°C. and a furnace pressure of 3 to 5 MPa, and subsequently cooled to roomtemperature. After finishing cooling, the alumina crucible is taken outof the furnace. As a result of the above procedure, a brown GaN singlecrystal layer with a thickness of 300 to 3000 μm is deposited on thesurface of the MOCVD-GaN template.

The GaN single crystal layer which has been obtained in such a manner isground with diamond abrasive grains to planarize a surface thereof. TheFlux-GaN template having the GaN single crystal layer formed on theMOCVD-GaN template is thereby obtained. However, the grinding isperformed to the extent that a total thickness of a nitride layer on theFlux-GaN template is sufficiently larger than a targeted thickness ofthe free-standing substrate 1 to be obtained eventually.

Subsequently, by a laser lift off method in which laser light is emittedfrom a side of the seed substrate to perform scanning at a scan speed of0.1 to 100 mm/sec., the seed substrate is separated from the Flux-GaNtemplate. Third harmonic of Nd: YAG having a wavelength of 355 nm, forexample, is preferably used as the laser light. In the above case, apulse width may be set to approximately 1 to 1000 ns and a pulse periodmay be set to approximately 1 to 200 kHz. In emitting the laser light,it is preferable to appropriately collect the laser light to adjust thelight density. It is preferable to emit the laser light while heatingthe Flux-GaN template from a side opposite to the seed substrate at atemperature of approximately 30 to 600° C.

After separating the seed substrate, a grinding processing is performedon a surface, from which the seed substrate has been detached, of alaminated structure which has been obtained. The free-standing substrate1 formed of GaN containing Zn doped at a concentration of 1×10¹⁸ cm⁻³ ormore is thereby obtained.

(Manufacture of Epitaxial Substrate)

Subsequently, a manufacture of the epitaxial substrate 10 by the MOCVDmethod is described. The epitaxial substrate 10 is obtained bylaminating the buffer layer 2, the channel layer 3, and the barrierlayer 4 in this order under the following condition in a state where thefree-standing substrate 1 is disposed on a susceptor provided in thereactor in the MOCVD furnace. However, described as an example of thebuffer layer 2 is a case where a single GaN buffer layer or amulti-layered buffer layer or a composition gradient buffer layercontaining Ga and Al as a group 13 element is formed. The formationtemperature means a susceptor heating temperature.

In the present embodiment, a gas ratio of group 15 to group 13 is aratio (molar ratio) of a supply amount of ammonia, which is a group 15(N) source, to a total supply amount of trimethylgallium (TMG),trimethylaluminum (TMA), and trimethylindium (TMI), which are group 13(Ga, Al, and In) sources. A gas ratio of an Al source gas to a group 13source gas in a case of making the barrier layer 4 of AlGaN is a ratio(molar ratio) of a supply amount of an Al source to a supply amount ofwhole group 13 (Ga, Al) sources, and a gas ratio of an In source gas toa group 13 source gas in a case of making the barrier layer 4 of InAlNis a ratio (molar ratio) of a supply amount of an In source to a supplyamount of whole group 13 (In, Al) sources. Both are defined inaccordance with a composition (an Al molar ratio x or an In compositionratio y) of a desired barrier layer 4.

Buffer Layer 2:

Formation temperature=1000 to 1200° C.;

Reactor pressure=15 to 105 kPa;

Carrier gas=hydrogen;

Gas ratio of group 15 to group 13=250 to 10000;

Gas ratio of Al source gas to group 13 source gas=0 (in a case of theGaN buffer layer);

Gas ratio of Al source gas to group 13 source gas=ranging from 0 to 1 inaccordance with a position in the thickness direction (in a case of themulti-layered buffer layer or the composition gradient buffer layer).

Channel Layer 3:

Formation temperature=1000 to 1150° C.;

Reactor pressure=15 to 105 kPa;

Carrier gas=hydrogen;

Gas ratio of group 15 to group 13=1000 to 10000.

Barrier layer 4 (in a case of formed of AlGaN):

Formation temperature=1000 to 1200° C.;

Reactor pressure=1 to 30 kPa;

Gas ratio of group 15 to group 13=5000 to 20000;

Carrier gas=hydrogen;

Gas ratio of Al source gas to group 13 source gas=0.1 to 0.4.

Barrier Layer 4 (in a Case of Formed of InAlN):

Formation temperature=700 to 900° C.;

Reactor pressure=1 to 30 kPa;

Gas ratio of group 15 to group 13=2000 to 20000;

Carrier gas=nitrogen;

Gas ratio of In source gas to group 13 source gas=0.1 to 0.9.

(Manufacture of HEMT Element)

The HEMT element 20 using the epitaxial substrate 10 can be manufacturedby applying a publicly known technique.

For example, after an element separation processing of removing aportion of a boundary between individual elements by performing etchingto approximately 50 to 1000 nm, using a photolithography process and aRIE method, an SiO₂ film having a thickness of 50 to 500 nm is formed ona surface of the epitaxial substrate 10 (the surface of the barrierlayer 4), and then the SiO₂ film at locations where the source electrode5 and the drain electrode 6 are to be formed is removed by etching usingthe photolithography process, thereby obtaining an SiO₂ pattern layer.

Next, a metal pattern of Ti/Al/Ni/Au is formed at the locations wherethe source electrode 5 and the drain electrode 6 are to be formed by thevacuum deposition method and the photolithography process, therebyforming the source electrode 5 and the drain electrode 6. The metallayers preferably have thicknesses of 5 to 50 nm, 40 to 400 nm, 4 to 40nm, and 20 to 200 nm in this order, respectively.

Subsequently, a thermal treatment is performed for 10 to 1000 seconds ina nitrogen gas atmosphere at a temperature of 600 to 1000° C. to improvethe ohmic contact of the source electrode 5 and the drain electrode 6.

Then, the SiO₂ film at locations where the gate electrode 7 is to beformed is removed from the SiO₂ pattern layer using the photolithographyprocess.

Furthermore, a Schottky metal pattern of Ni/Au is formed at a locationfor formation on the gate electrode 7 by the vacuum deposition methodand the photolithography process, thereby forming the gate electrode 7.The metal layers preferably have thicknesses of 4 to 40 nm and 20 to 200nm, respectively.

The HEMT element 20 is obtained by the processes described above.

<Uneven Distribution of Si and Diffusion of Zn>

In the HEMT element 20 manufactured under the procedures and theconditions described above, when a region consisting of thefree-standing substrate 1 and the buffer layer 2 is defined as a firstregion, part of the first region has a second region containing Si at ahigh concentration of 1×10¹⁷ cm⁻³ or more. Since Si is not purposelycontained in the process of manufacturing the HEMT element 20,particularly in the process of manufacturing the free-standing substrate1 and adjacently forming the buffer layer 2 on the free-standingsubstrate 1, containing Si at the concentration described above in thesecond region is considered to be a result that Si, which had been takenfrom outside halfway through the process, remains after the formation ofthe HEMT element 20. More specifically, the second region includes theinterface between the free-standing substrate 1 and the buffer layer 2.However, the second region is not formed in the free-standing substrate1.

In addition, in the HEMT element 20 according to the present embodiment,Zn, with which the free-standing substrate 1 is doped, is diffused intoat least the buffer layer 2. Furthermore, Zn is contained in the wholerange of the second region described above at the concentration of1×10¹⁷ cm⁻³ or more (Zn is contained so that a minimum value of theconcentration in the second region is 1×10¹⁷ cm⁻³).

In the HEMT element 20 according to the present embodiment, as a resultthat the above condition of the concentration is satisfied, the leakagecurrent is reduced in driving, and a high breakdown voltage (highelement voltage) is achieved.

In the meanwhile, in the HEMT element having the minimum value of the Znconcentration in the second region smaller than 1×10¹⁷ cm⁻³, it isconfirmed that the leakage current increases and the breakdown voltageis low.

Si taken from outside may function as the donor element, and when thediffusion of Zn satisfying the concentration condition described abovedoes not occur, the conductive layer which serves as the leakage pathwayof the drain-source current is formed in the HEMT element by thefunction, and there is a possibility of the reduction in the pinch-offcharacteristics and the reduction of the breakdown voltage. However,when Zn is contained so that the minimum value of the concentration inthe second region is 1×10¹⁷ cm⁻³, it is considered that Zn inhibits Sifunctioning as the donor element.

That is to say, according to the present embodiment, the semiconductorelement in which the leakage current in driving the semiconductorelement is reduced and the breakdown voltage (the element voltage) isenhanced can be obtained.

EXAMPLES Example 1

Manufacture of Zn-Doped GaN Single Crystal Substrate by a Flux Method

A GaN low-temperature buffer layer is formed to have a thickness of 30nm at a temperature of 550° C. on a surface of a c-plane sapphiresubstrate having a diameter of 2 inches and a thickness of 0.43 mm, andsubsequently, a GaN thin film having a thickness of 3 μm is formed bythe MOCVD method at a temperature of 1050° C., thereby obtaining aMOCVD-GaN template usable as a seed substrate.

A Zn-doped GaN single crystal layer is formed by the Na flux methodusing the MOCVD-GaN template which has been obtained as the seedsubstrate.

Specifically, the MOCVD-GaN template was firstly disposed in an aluminacrucible, and subsequently, the alumina crucible was filled with 30 g ofmetal Ga, 45 g of metal Na, 1 g of metal Zn, and 100 mg of C. Thealumina crucible was put in a heating furnace and heated forapproximately 100 hours with a furnace temperature of 850° C. and afurnace pressure of 4.5 MPa, and subsequently cooled to roomtemperature. When the alumina crucible was taken out of the furnaceafter finishing cooling, a brown GaN single crystal layer was depositedon the surface of the MOCVD-GaN template with a thickness ofapproximately 1000 μm.

The GaN single crystal layer which had been obtained in such a mannerwas ground with diamond abrasive grains so that the surface thereof wasplanarized and the nitride layer formed on a base substrate had a totalthickness of 900 μm. The Flux-GaN template having the GaN single crystallayer formed on the MOCVD-GaN template was thereby obtained. When theFlux-GaN template was viewed with the naked eye, no cracking was foundthereon.

Subsequently, by a laser lift-off method in which laser light wasemitted from a side of the seed substrate to perform scanning at a scanspeed of 30 mm/sec., the seed substrate was separated from the Flux-GaNtemplate. Third harmonic of Nd: YAG having a wavelength of 355 nm wasused as the laser light. A pulse width was set to approximately 30 nsand a pulse period was set to approximately 50 kHz. In emitting thelaser light, the laser light was collected to have a circular shape witha diameter of approximately 20 μm, thereby having a light density ofapproximately 1.0 J/cm. The laser light was emitted while heating theFlux-GaN template from a side opposite to the seed substrate at atemperature of around 50° C.

After separating the seed substrate, a grinding processing was performedon a surface, from which the seed substrate had been detached, of alaminated structure which had been obtained, thereby obtaining aZn-doped GaN free-standing substrate having a total thickness of 430 μm.

The crystallinity of the Zn-doped GaN substrate which had been obtainedwas evaluated with an X-ray rocking curve. A half-value width of (0002)plane reflection was 120 seconds and a half-value width of (10-12) planereflection was 150 seconds, both showing favorable crystalline nature.

Manufacture of Epitaxial Substrate by MOCVD Method

Subsequently, an epitaxial substrate was manufactured by the MOCVDmethod. Specifically, a GaN layer as a buffer layer, a GaN layer as achannel layer, and an AlGaN layer as a barrier layer were laminated onthe Zn-doped GaN substrate described above in this order under thefollowing condition. In the present embodiment, a gas ratio of group 15to group 13 is a ratio (molar ratio) of a supply amount of a group 15(N) source to a supply amount of group 13 (Ga, Al) sources. A gas ratioof an Al source gas to a group 13 source gas is a ratio (molar ratio) ofa supply amount of an Al source to a supply amount of whole group 13(Ga, Al) sources.

GaN Buffer Layer:

Formation temperature=1150° C.;

Reactor pressure=15 kPa;

Gas ratio of group 15 to group 13=1000;

Thickness=600 nm.

GaN Channel Layer:

Formation temperature=1050° C.;

Reactor pressure=15 kPa;

Gas ratio of group 15 to group 13=1000;

Thickness=3000 nm.

AlGaN Barrier Layer:

Formation temperature=1050° C.;

Reactor pressure=5 kPa;

Gas ratio of group 15 to group 13=12000;

Gas ratio of an Al source gas to a group 13 source gas=0.25;

Thickness=25 nm.

After the formation of the layers described above, the susceptortemperature was lowered to around room temperature, and the internal gasof the reactor was returned to atmospheric pressure. Then, the epitaxialsubstrate which had been manufactured was taken out.

Manufacture of HEMT Element

Subsequently, the HEMT element 20 was manufactured using the epitaxialsubstrate 10. The HEMT element was designed to have a gate width of 100μm, a source-gate distance of 1 μm, a gate-drain distance of 10 μm, anda gate length of 1 μm.

Firstly, a portion of a boundary between individual elements was removedby etching to a depth of approximately 100 nm using the photolithographyprocess and the RIE method.

Next, the SiO₂ film having a thickness of 100 nm was formed on theepitaxial substrate, and then the SiO₂ film at locations where thesource electrode and the drain electrode were to be formed was removedby etching using the photolithography process, thereby obtaining an SiO₂pattern layer.

Next, a metal pattern of Ti/Al/Ni/Au (each having a film thickness of25/200/20/100 nm) was formed at the locations where the source electrodeand the drain electrode were to be formed by the vacuum depositionmethod and the photolithography process, thereby forming the sourceelectrode and the drain electrode. Subsequently, a thermal treatment isperformed for 30 seconds in a nitrogen gas atmosphere at a temperatureof 825° C. to improve the ohmic contact of the source electrode and thedrain electrode.

Then, the SiO₂ film at locations where the gate electrode was to beformed was removed from the SiO₂ pattern layer using thephotolithography process.

Furthermore, a Schottky metal pattern of Ni/Au (each having a filmthickness of 20/100 nm) was formed at the locations where the gateelectrode was to be formed by the vacuum deposition method and thephotolithography process, thereby forming the gate electrode.

The HEMT element was obtained by the processes described above.

SIMS Evaluation of HEMT Element

An element analysis in the depth direction was performed on the HEMTelement, which had been obtained, by secondary ion mass spectrometry(SIMS), and each concentration of the Zn element and the Si element inthe AlGaN barrier layer, the GaN channel layer, the GaN buffer layer,and the GaN substrate was examined.

FIG. 2 is drawing illustrating a concentration profile of Zn element andSi element in a neighborhood of an interface between a GaN buffer layerand a GaN substrate. A result illustrated in FIG. 2 shows the followingmatters.

(1) The GaN substrate is doped with the Zn element at a highconcentration (1×10¹⁹ cm⁻³).

(2) In a first region RE1 consisting of the GaN substrate and the GaNbuffer layer, a second region RE2 containing the Si element at a highconcentration of 1×10¹⁷ cm⁻³ or more is formed in a neighborhood of aninterface between the GaN substrate and the GaN buffer layer, and a peakconcentration of the Si element is 6×10¹⁸ cm⁻³.

(3) In the GaN buffer layer, the Zn concentration gradually decreasescompared to the Si concentration. That is to say, the Zn element issignificantly diffused compared to the Si element in the GaN bufferlayer.

(4) A minimum value of the Zn concentration in the second region RE2 is5.3×10¹⁷ cm⁻³ (10¹⁷ cm⁻³).

Electrical Characteristics Evaluation of HEMT Element

Drain current-drain voltage characteristics (Id-Vd characteristics) ofthe HEMT element were evaluated in a DC mode using a semiconductorparameter analyzer. A pinch-off threshold voltage was Vg=−3V.

Adopted as an index for evaluating the drain current leakage amount atthe time of pinch-off was the drain current Id_(Vd=10V·Vg=−10V) in acase of applying the drain voltage Vd=10V and the gate voltage Vg=−10V.The drain current Id_(Vd=10V·Vg=−10V) was calculated to be 3×10⁻⁷ A inthe case of the HEMT element according to the present Example.Id_(Vd=10V·Vg=−10V) is preferably as small as possible, and it can bedetermined that the drain current leakage amount is small whenId_(Vd=10V·Vg=−10V)≤1×10⁻⁵ A/mm is satisfied in a value normalized withthe gate width. In the case of the HEMT element according to the presentExample, the drain current leakage amount normalized with the gate widthof 100 μm is calculated to be 3×10⁻⁶ A/mm, thus is determined to besufficiently small.

Subsequently, an element breakdown voltage was measured. Adopted as anindex for evaluating the element breakdown voltage was the drain voltageVdb in which the drain current Id exceeded 1×10⁻⁵ A for the first time(1×10⁻⁴ A/mm in case of being normalized with the gate width of 100 μm)when the drain voltage Vd was gradually increased from 0V while applyingthe gate voltage Vg=−10V. The drain voltage was calculated to be 850V inthe case of the HEMT element according to the present Example. Vdb ispreferably as large as possible, and it can be determined that theelement breakdown voltage is sufficient when Vdb≥300V is satisfied, thusthe element breakdown voltage of the HEMT element according to thepresent Example is determined to be sufficiently large.

Comparative Example 1

The HEMT element was manufactured under conditions similar to that ofExample 1 except that the growth condition of the GaN buffer layer wasas follows, differing from Example 1.

GaN Buffer Layer:

Formation temperature=1050° C.;

Reactor pressure=15 kPa;

Gas ratio of group 15 to group 13=1000;

Thickness=600 nm.

FIG. 3 illustrates a concentration profile of the Zn element and the Sielement in the neighborhood of the interface between the GaN bufferlayer and the GaN substrate obtained by performing the SIMS measurementon the HEMT element, which has been obtained, under conditions similarto that of Example 1. A result illustrated in FIG. 3 shows the followingmatters.

(1) The GaN substrate is doped with the Zn element at a highconcentration in the manner similar to the Example 1.

(2) In the first region RE1 consisting of the GaN substrate and the GaNbuffer layer, the second region RE2 is formed in the neighborhood of theinterface between the GaN substrate and the GaN buffer layer in themanner similar to Example 1.

(3) In the GaN buffer layer, the Zn concentration comparatively sharplydecreases compared to the Si concentration, differing from Example 1.That is to say, the diffusion of the Zn element is suppressed comparedto the diffusion of the Si element in the GaN buffer layer.

(4) A minimum value of the Zn concentration in the second region RE2 is1.7×10¹⁵ cm⁻³ (<1×10¹⁷ cm⁻³).

Id_(Vd=10V·Vg=−10V) of the HEMT element was calculated to be 8×10⁻⁵ A(8×10⁻⁴ A/mm in case of being normalized with the gate width of 100 μm)under the condition similar to that of the Example 1. That is to say, itwas confirmed that the drain current leakage amount was large, and theHEMT element according to the present Comparative Example did not havesufficient pinch-off characteristics.

Vdb was calculated to be 100V under conditions similar to that of theExample 1, thus sufficient element breakdown voltage was not obtained.

Examples 2 to 6, Comparative Examples 2 to 3

The HEMT elements were manufactured under conditions similar to that ofExample 1 except that the growth conditions of the GaN buffer layer (thegrowth temperature, the reactor pressure, the gas ratio of group 15 togroup 13, and the formation thickness), for example, were variouslychanged. Then, the distribution in the depth direction of the Znconcentration and the Si concentration in the HEMT element, which hadbeen obtained, was obtained by the SIMS measurement, and theId_(Vd=10V·Vg=−10V) and Vdb were measured.

A list of the results thereof is shown by Table 1 together with theresults of Example 1 and Comparative Example 1.

TABLE 1 Growth condition of GaN buffer layer SIMS measurement GasMinimum value of ratio Zn concentration of Leakage Element in regionwhere Si group current breakdown concentration is Growth Reactor 15 toevaluation voltage equal to or larger temperature pressure groupThickness Id_(vd=10v·vg−10v) Vdb than [° C.] [kPa] 13 [nm] [A/mm] [V] 1× 10¹⁷ cm⁻³ [cm⁻³] Example 1 1150 15 1000 600 3 × 10⁻⁶ 850 5.3 × 10¹⁷Example 2 1100 15 1000 600 4 × 10⁻⁶ 820 5.3 × 10¹⁷ Comparative 1050 151000 600 8 × 10⁻⁴ 100 1.7 × 10¹⁵ Example 1 Example 3 1050 30 1000 600 6× 10⁻⁶ 620 1.6 × 10¹⁷ Example 4 1050 45 1000 600 6 × 10⁻⁶ 690 3.6 × 10¹⁷Example 5 1050 15 2000 600 9 × 10⁻⁶ 450 1.0 × 10¹⁷ Example 6 1050 154000 600 4 × 10⁻⁶ 500 4.1 × 10¹⁷ Comparative 1050 15 1000 300 1 × 10⁻³85 2.3 × 10¹⁵ Example 2 Comparative 1050 15 1000 1200 7 × 10⁻³ 115 5.1 ×10¹⁵ Example 3As indicated by Table 1, in the cases of Examples 1 to 6, manufacturedunder the condition where the minimum value of the concentration of Znin the region RE2 was equal to or larger than. 1×10¹⁷ cm⁻³, the HEMTelement having the small drain current leakage amount(Id_(Vd=10V·Vg=−10V)≤1×10⁻⁵ A/mm) and the large element breakdownvoltage (Vdb≥300V) could be obtained. In the meanwhile, in the cases ofComparative Examples 1 to 3 manufactured under the condition where theminimum value of the concentration of Zn in the region RE2 was smallerthan 1×10¹⁷ cm⁻³, only the HEMT element having a large drain currentleakage amount and small element breakdown voltage could be obtained.

Example 7

The epitaxial substrate 10 and, further, the HEMT element 20 weremanufactured under conditions similar to that of Example 1 except thatthe growth condition of the buffer layer 2 and the channel layer 3 wasas follows, differing from Example 1. In the formation of the bufferlayer 2 in the above processing, the formation condition is set in twostages of a first condition and a second condition, and is switched fromthe first condition to the second condition halfway through theformation. This is intended to form the buffer layer 2 as themulti-layered buffer layer in which the GaN layer is laminated on theAl_(a)Ga_(1-a)N layer (0<a≤1) or the composition gradient buffer layerin which the existence ratio of Al and Ga is different in the thicknessdirection. The total thickness of the buffer layer 2 was set to 110 nm.

Buffer Layer (First Condition):

Formation temperature=1050° C.;

Reactor pressure=5 kPa;

Group 13 source gas=Al source and Ga source;

Gas ratio of group 15 to group 13=2000;

Gas ratio of Al source gas to group 13 source gas=0.03;

Growth rate=1 nm/sec.;

Growth time=10 seconds.

Buffer Layer (Second Condition):

Formation temperature=1050° C.;

Reactor pressure=10 kPa;

Group 13 source gas=Ga source;

Gas ratio of group 15 to group 13=500;

Growth rate=1 nm/sec.;

Growth time=100 seconds.

GaN Channel Layer:

Formation temperature=1050° C.;

Reactor pressure=100 kPa;

Gas ratio of group 15 to group 13=2000;

Thickness=900 nm.

FIG. 4 is a drawing illustrating a concentration profile of the Znelement and the Si element from the surface (upper surface) of thebarrier layer 4 in the depth direction obtained by performing themeasurement on the HEMT element, which has been obtained, in the depthdirection by the SIMS measurement under conditions similar to that ofExample 1 and a secondary ion signal profile of the Al element in thedepth direction (a distribution of a secondary ion count rate of the Alelement in the depth direction). The result illustrated in FIG. 4 showsthe following matters.

(1) The GaN substrate is doped with the Zn element at a highconcentration.

(2) In the first region RE1 consisting of the GaN substrate and thebuffer layer, the second region RE2 containing the Si element at thehigh concentration of 1×10¹⁷ cm⁻³ or more is formed in the neighborhoodof the interface between the GaN substrate and the buffer layer, and thepeak concentration of the Si element is 3×10¹⁸ cm⁻³.

(3) The Si concentration has a peak in the second region RE2 and sharplydecreases with a decreasing distance to the channel layer, however, thereduction in the Zn concentration from the buffer layer to the channellayer is gradual. That is to say, the Zn element is significantlydiffused compared to the Si element. Specifically, in the channel layer,the Zn element is diffused to the range of 200 to 250 nm from theinterface between the channel layer and the buffer layer (the interfacebetween the channel layer and the first region RE1).

(4) A minimum value of the Zn concentration in the second region RE2 is5.3×10¹⁷ cm⁻³ (≥1×10¹⁷ cm⁻³).

(5) The Al element is contained in a range wider than 110 nm, which is atargeted thickness of the whole buffer layer, and the range alsoincludes part of the GaN substrate.

Id_(Vd=10V·Vg=−10V) of the HEMT element was calculated to be 8×10⁻⁸ A(8×10⁻⁷ A/mm in the case of being normalized with the gate width of 100μm) under conditions similar to that of Example 1. That is to say, itwas confirmed that the drain current leakage amount was small, and theHEMT element according to the present Example had favorable pinch-offcharacteristics.

Vdb was calculated to be 1200V under conditions similar to that ofExample 1, thus a sufficient element breakdown voltage was obtained.

Example 8

The HEMT element 20 was manufactured under conditions similar to that ofExample 7 except that the growth condition of the buffer layer 2 and thechannel layer 3 was as follows, differing from Example 7. That is tosay, also in the present Example, in the formation of the buffer layer2, the formation condition is set in two stages of a first condition anda second condition, and is switched from the first condition to thesecond condition halfway through the formation. The total thickness ofthe buffer layer 2 was set to 350 nm.

Buffer Layer (First Condition):

Formation temperature=1050° C.;

Reactor pressure=5 kPa;

Group 13 source gas=Al source and Ga source;

Gas ratio of group 15 to group 13=2000;

Gas ratio of Al source gas to group 13 source gas=0.01;

Growth rate=1 nm/sec.;

Growth time=50 seconds.

Buffer Layer (Second Condition):

Formation temperature=1050° C.;

Reactor pressure=10 kPa;

Gas ratio of group 15 to group 13=500;

Growth rate=1 nm/sec.;

Growth time=300 seconds.

GaN Channel Layer:

Formation temperature=1050° C.;

Reactor pressure=100 kPa;

Gas ratio of group 15 to group 13=2000;

Thickness=1700 nm.

A concentration profile of the Zn element and the Si element from thesurface (upper surface) of the barrier layer 4 in the depth directionobtained by performing the SIMS measurement on the HEMT element, whichhas been obtained, under conditions similar to that of Example 1 and asecondary ion signal profile of the Al element in the depth directionshow the following matters.

(1) The GaN substrate is doped with the Zn element at a highconcentration (1×10¹⁹ cm⁻³).

(2) In the first region RE1 consisting of the GaN substrate and thebuffer layer, the second region RE2 containing the Si element at thehigh concentration of 1×10¹⁷ cm⁻³ or more is formed in the neighborhoodof the interface between the GaN substrate and the buffer layer, and thepeak concentration of the Si element is 4×10¹⁸ cm⁻³.

(3) The Si concentration has a peak in the second region RE2 and sharplydecreases with a decreasing distance to the channel layer, however, thereduction in the Zn concentration from the buffer layer to the channellayer is gradual. That is to say, the Zn element is significantlydiffused compared to the Si element.

(4) A minimum value of the Zn concentration in the second region RE2 is8.2×10¹⁷ cm⁻³ (≥1×10¹⁷ cm⁻³).

(5) The Al element is contained in a range wider than 350 nm which is atargeted thickness of the whole buffer layer, and the range alsoincludes part of the GaN substrate.

Id_(Vd=10V·Vg=−10V) of the HEMT element was calculated to be 2×10⁻⁷ A(2×10⁻⁶ A/mm in the case of being normalized with the gate width of 100μm) under conditions similar to that of Example 1. That is to say, itwas confirmed that the drain current leakage amount was small, and theHEMT element according to the present Example had favorable pinch-offcharacteristics.

Vdb was calculated to be 1050V under conditions similar to that ofExample 1, thus a sufficient element breakdown voltage was obtained.

The invention claimed is:
 1. An epitaxial substrate for semiconductor elements, comprising: a semi-insulating free-standing substrate formed of GaN doped with Zn; a buffer layer formed of a group 13 nitride adjacent to said free-standing substrate; a channel layer formed of a group 13 nitride adjacent to said buffer layer; and a barrier layer formed of a group 13 nitride on an opposite side of said buffer layer with said channel layer therebetween, wherein part of a first region consisting of said free-standing substrate and said buffer layer is a second region containing Si at a concentration of 1×10¹⁷ cm⁻³ or more, and a minimum value of a concentration of Zn in said second region is 1×10¹⁷ cm⁻³.
 2. The epitaxial substrate for the semiconductor elements according to claim 1, wherein said buffer layer is formed of GaN, said channel layer is formed of GaN, and said barrier layer is formed of AlGaN.
 3. The epitaxial substrate for the semiconductor elements according to claim 1, wherein said buffer layer is either of a multi-layered buffer layer which is formed by laminating two or more group 13 nitride layers having different compositions, or a composition gradient buffer layer formed of a group 13 nitride containing two or more group 13 elements, each element having an existence ratio changed in a thickness direction, said channel layer is formed of GaN, and said barrier layer is formed of AlGaN.
 4. The epitaxial substrate for the semiconductor elements according to claim 1, wherein said second region exists in said first region, including an interface between said free-standing substrate and said buffer layer.
 5. The epitaxial substrate for the semiconductor elements according to claim 4, wherein said buffer layer is formed of GaN, said channel layer is formed of GaN, and said barrier layer is formed of AlGaN.
 6. The epitaxial substrate for the semiconductor elements according to claim 4, wherein said buffer layer is either of a multi-layered buffer layer which is formed by laminating two or more group 13 nitride layers having different compositions, or a composition gradient buffer layer formed of a group 13 nitride containing two or more group 13 elements, each element having an existence ratio changed in a thickness direction, said channel layer is formed of GaN, and said barrier layer is formed of AlGaN.
 7. A semiconductor element, comprising: a semi-insulating free-standing substrate formed of GaN doped with Zn; a buffer layer formed of a group 13 nitride adjacent to said free-standing substrate; a channel layer formed of a group 13 nitride adjacent to said buffer layer; a barrier layer formed of a group 13 nitride on an opposite side of said buffer layer with said channel layer therebetween; and a gate electrode, a source electrode, and a drain electrode provided on said barrier layer, wherein part of a first region consisting of said free-standing substrate and said buffer layer is a second region containing Si at a concentration of 1×10¹⁷ cm⁻³ or more, and a minimum value of a concentration of Zn in said second region is 1×10¹⁷ cm⁻³.
 8. The semiconductor element according to claim 7, wherein said buffer layer is formed of GaN, said channel layer is formed of GaN, and said barrier layer is formed of AlGaN.
 9. The semiconductor element according to claim 7, wherein said buffer layer is either of a multi-layered buffer layer which is formed by laminating two or more group 13 nitride layers having different compositions, or a composition gradient buffer layer formed of a group 13 nitride containing two or more group 13 elements, each element having an existence ratio changed in a thickness direction, said channel layer is formed of GaN, and said barrier layer is formed of AlGaN.
 10. The semiconductor element according to claim 7, wherein said second region exists in said first region, including an interface between said free-standing substrate and said buffer layer.
 11. The semiconductor element according to claim 10, wherein said buffer layer is formed of GaN, said channel layer is formed of GaN, and said barrier layer is formed of AlGaN.
 12. The semiconductor element according to claim 10, wherein said buffer layer is either of a multi-layered buffer layer which is formed by laminating two or more group 13 nitride layers having different compositions, or a composition gradient buffer layer formed of a group 13 nitride containing two or more group 13 elements, each element having an existence ratio changed in a thickness direction, said channel layer is formed of GaN, and said barrier layer is formed of AlGaN. 